The invention relates generally to lightbulb packages and more particularly to a lightbulb package that utilizes a phosphor light emitting diode as the light source.
Common lightbulb packages utilize a light source that includes an incandescent filament within a glass enclosure. However, these glass enclosures are fragile and, as such, can easily break even when subjected to only a moderate impact. In addition, the incandescent filaments themselves are fragile and tend to gradually degrade during use, such that the useful light output generated by the filaments decreases over time. The increasing fragility of the filament with age eventually leads to breakage. Typical incandescent lightbulbs have a mean life of 500 to 4,000 hours, which means that half of a population of lightbulbs will fail in that time because of filament breakage.
With reference to FIG. 1, a conventional halogen lightbulb package 10 of MR-16 outline type is shown. The halogen lightbulb package includes a halogen bulb 12 positioned in the center of a reflector 14, which functions to direct the light produced by the halogen bulb in a generally uniform direction. The package further includes a pair of output terminals 16 and 18 to receive electrical power. The front open face of the package may be protected with a dust cover (not shown). A disadvantage of the package of FIG. 1 is the use of the halogen bulb as the light source. As previously described, the fragility of the glass enclosure and the incandescent filament limits the operating life of the halogen bulb.
Confronted with the above disadvantage, the use of light emitting diodes as a potential light source in a lightbulb package has been examined. Light emitting diodes (LEDs) are well-known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. Traditionally, the most efficient LEDs emit light having a peak wavelength in the red region of the light spectrum, i.e., red light. However, a type of LED based on Gallium Nitride (GaN) has recently been developed that can efficiently emit light having a peak wavelength in the blue region of the spectrum, i.e., blue light. This new type of LED can provide significantly brighter output light than traditional LEDs.
In addition, since blue light has a shorter peak wavelength than red light, the blue light generated by the GaN-based LEDs can be more readily converted to produce light having a longer peak wavelength. It is well known in the art that light having a first peak wavelength (the xe2x80x9cprimary lightxe2x80x9d) can be converted into light having a longer peak wavelength (the xe2x80x9csecondary lightxe2x80x9d) using a process known as fluorescence. The fluorescent process involves absorbing the primary light by a photoluminescent phosphor material, which excites the atoms of the phosphor material, and emitting the secondary light. An LED that utilizes the fluorescent process is defined herein as a xe2x80x9cphosphor LED.xe2x80x9d The peak wavelength of the secondary light will depend on the phosphor material. The combined light of unconverted primary light and the secondary light produces the output light of the phosphor LED. Thus, the particular color of the output light will depend on the spectral distributions of the primary and second lights. Consequently, a lightbulb package can be configured to generate white output light by selecting an appropriate phosphor material for the GaN-based LED.
U.S. Pat. No. 5,813,753 to Vriens et al. describes a light emitting device having an LED as the light source that utilizes phosphor grains dispersed in an epoxy layer to transform the color of the light emitted by the LED into a desired color. The phosphor grains are described as a single type of phosphor material or a mixture of different phosphor materials, depending on the desired color of the output light. A concern with the use of an epoxy layer that includes phosphor grains as described in Vriens et al. is the difficulty in dispensing the phosphor grains in a repeatable and uniform manner. Such difficulty leads to a population of finished devices having variable performances, i.e., the color of the output light may vary from one finished device to another.
In light of the above concern, what is needed is a lightbulb package having a phosphor LED as the light source that can generate output light of a prescribed color and a method of fabricating such a lightbulb package.
An LED package and a method of fabricating the LED package utilize a prefabricated fluorescent member that contains a fluorescent material that can be separately tested for optical properties before assembly to ensure the proper performance of the LED package with respect to the color of the output light. The LED package includes one or more LED dies that operate as the light source of the package. Preferably, the fluorescent material included in the prefabricated fluorescent member and the LED dies of the LED package are selectively chosen, so that output light generated by the LED package duplicates natural white light.
In a first embodiment of the invention, the LED package includes four 3 volt gallium nitride-based LED dies that are individually mounted on separate reflector cups, which are attached to a leadframe. In this embodiment, the LED package is configured to be interchangeable with an industry standard MR-16 halogen outline package. However, the LED package may be configured to resemble other industry standard packages, such as MRC-11, MRC-16, PAR-36, PAR-38, PAR-56 and PAR-64. In fact, the LED package may be configured in a completely different lightbulb outline package.
Also attached to the leadframe are output terminals that provide electrical power to the LED dies. The LED dies are electrically connected to the terminals in a specific configuration. In one exemplary configuration, the LED dies are connected in series, so that the overall forward voltage of the package is 12 volts. In an alternative exemplary configuration, the LED dies are connected in series and parallel to create a 6 volt device. The exact electrical configuration of the LED dies, as well as the voltage of the LED dies, are not critical to the invention. Furthermore, the number of LED dies included in the LED package is not critical to the invention.
Deposited over the LED dies is an encapsulant material. The encapsulant material may be epoxy or other suitable transparent material. Preferably, the encapsulant material is an optical grade silicone gel, since silicone gel can withstand exposure to high temperatures without degradation. In addition, silicone gel having a refractive index of 1.5 is currently available, which results in an efficient extraction of light generated by the LED dies.
The prefabricated fluorescent member of the LED package is affixed over the encapsulant material. In this embodiment, the prefabricated fluorescent member is a substantially planar disk that is optically transparent. However, the fluorescent member may be configured in another shape, such as a square or a rectangle, depending on the specification of the LED package. As previously noted, the fluorescent material contained in the prefabricated fluorescent member can be chosen to produce white light. As an example, the fluorescent material may include gadolinium doped, cerium activated yttrium aluminum garnet phosphor grains.
The LED package further includes a lens that is attached to the prefabricated fluorescent member and a reflector that is positioned over the lens. The lens and the reflector ensure that most of the light energy generated by the LED package is output generally along a common direction.
In a second embodiment of the invention, the lens of the LED package is a concave lens and the prefabricated fluorescent member is formed in the inner surface of the concave lens. As such, the prefabricated fluorescent member conforms to the contour of the inner surface of the concave lens. In this embodiment, the optical properties of the fluorescent member can be tested by examining the lens and the attached fluorescent member as a single component.
The method of fabricating the LED package in accordance with the invention includes forming a number of transparent fluorescent members. In a first embodiment, the fluorescent members may be substantially planar plates, such as disks. These plates may be formed by cutting sheets of silicone rubber into the desired shapes. In a second embodiment, the fluorescent members may be non-planar disks that conform to the inner surface of a concave lens. These non-planar disks may be formed by allowing an optically transparent material, such as silicone, polycarbonate or acrylic, to be molded onto the inner surface of a concave lens. Next, the fluorescent members are tested for optical properties. As an example, the fluorescent members may be tested using a monochromatic standard source to activate the phosphor and then measuring the characteristics of the output from the fluorescent members. The tested fluorescent members can then be categorized for a set of optical properties.
After the fluorescent members are tested, one or more GaN-based LED dies are mounted onto a leadframe. Next, a transparent encapsulant material is deposited over the mounted LED dies. Preferably, the encapsulant material is an optical grade silicone gel, which has a high thermal stability and has a desired refractive index for an efficient light extraction. A fluorescent member having a predefine set of optical properties is then placed over the encapsulant material. Next, a lens is attached to the fluorescent member. This step is not applicable for the second embodiment. A reflector is then mounted over the lens. After the reflector has been mounted, a dust cover may be attached to the rim of the reflector to complete the LED package.
An advantage of the invention is that the fluorescent member can be tested prior to assembly which ensures that the finished device will have specific optical properties, thereby reducing production costs that are associated to fabrication of unwanted devices, i.e., devices that do not meet the desired specifications.